273 camshaft selection help

No, the replacement 340 cam is;
20-309-4 [7]
http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=643&sb=2

The XE268 is;
20-223-3
http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=627&sb=2

The one you mention is my 3rd choice. More torque with that cam, slightly less HP.